Method of and apparatus for producing a straight bore cold drawn tube

ABSTRACT

An arrangement of apparatus for producing a straight bore cold drawn tube, where the internal surface of the finished tube has a minimum of surface wave. This is accomplished by extending the mandrel to a selected position downstream of the die, and limiting the stiffness ratio between tube and mandrel.

[73] Assignee: The Babcock 8: Wilcox Company,

1 [22 Filedz United States Pat'ent [1911 Evans n11 3,709,020 1 Jan.9,1973

[54] METHOD OF AND APPA ATUS FOR I PRODUCING ASTRAIGHT BORE COLD DRAWN TUBE inventor: sid'i 67mins, Beaver Fang-e New York, NY.

I r v y 14,1970

1 ppl. No.: 317,243

521' U.S.Cl. 72/2s3, 72/370.

[51] Inn-c1. ..-.B211/24,B2'ib17/02 ["58] Rie orse reh ..;..1. .;;..r;..7 2/283,274,37or- [56] References Cited UNITED-STATES PATENTS 3,570,297. 3mm ..Matthev s...;

693,119 2/1902 Diescher Kobberup et a1 "72/283 3,327,513 6/1967 Hinshaw .1 .-..72/367 2,258,242 10/1941 Ditzel et a1. .12/283 3,564,884 2/1971 Flirishaw 72/285 1,967,487 7/1934 Waisnei ......:::72/283 Primary Examiner-*Richard-L Herbst Assistant Examiner-Michael I. Keenan Attorney-J. Mag'uire 57 ABSTRACT An arrangement of apparatus for producing a straight bore cold drawn tube, where the internal surface of the finished tube has a minimum of surface wave. This is accomplished by extending the mandrel to a selected position downstream of the die/and limiting v the stiffness ratio between tube and mandrel.

2 Claims, 6 Drawing Figures PATENTEDJAH 9 I973 SHEET 1 OF 2 FIG.1A

FIG.IB

T\ TY ISCL FIGLS 1O CONTACT LENGTH OF TUBE TO MANDREL-/ OF SPIRAL PITCH THOUSANDTHS /15" PATENTEDJAN 9 I973 3.709.020

SHEET 2 0F 2 FIG.2

M v v 10 V TUBE LENGTH-[ NCHES 210 'INVENTOR.

Sidley. 0. Evans 0 E A'T ORNEY t The present invention relates to a method of and apparatus for cold drawing a straight bore tube, and more particularly to the cold drawing of steel tubes to tolerances of internal straightness and diameter.

- In the production of seamless steel tubes, it is customary to pierce a billet and to then subject the hollow to cold drawing procedures to form commercial tubular products. In any piercing operation, factors can arise that will cause the tube to have a heavier wall on one side than on the opposite side. This displaces the centerline of the inside surface from the centerline of the outside surface and is known as eccentricity.

The most common method of piercing a billet is known as rotary piercing, which consists of crossrolling over a tapered tool. In this process, the eccen- -cial measures are taken, both outside and inside will generally have spiral waves which produce the displacement between their centerlines. I

Commercially produced tubes are used for many purposes where a slight wave in inner and outer surfaces is inconsequential in so far as the use of the tube is concerned. I-Iowever,.in some uses, departure from I inside straightness is detrimental. For example, hydraulic or pneumatic cylinders cannottolerate any' appreciable departure from straightness of the inside surface because the resultant displacement of the piston from the centerline of the packing gland, which seals the piston rod, will result in binding and destruction of the cylinder. So important is the inside straightness that frequently tubes will be honed with special long hones so as to cut out the high points of the waves. This is a 'costly process involving honing to the base of the wave to get cleanup; but straightness is so important that the cost is justified. I A

In the present invention, it has been foundthat a tube can be given a normal cold drawn pass so that the spiral wave on the inside surface is minimized, and is transferred to the outside surface, whereit will not be detrimental in the manufacture of hydraulic or pneumatic cylinders. This is accomplished by theuse of a mandrel which is extended downstream in the tube drawing direction from the throat of the reducing die. The length of the downstream extension for a given tube must be greater than a minimum required for satisfactory straightening of the inside surface and less than the limit imposed by increasing friction forces exceeding the strength of the mandrel bar,

FIGS. 1A and 1B are sectional views of rotary pierced tubes showing the effectof the spiral eccentricity which is exaggerated for. illustration,. with FIG. 1A showing the normal-rotary piercedtube with the spiral surface wave on both the inside and outside surfaces, and with FIG. 18 showing astraig'ht bore tube with the spiral surface wave alltransferred to the outside surface. A

FIG. 2 is a schematic elevation, partly in section, of a operation so as to produce a straight bore tube according to the invention,

FIG. 3 is similar to FIG. 2 showing the relationship of the die and mandrel during the straight bore tube drawing operation after the mandrel has traveled forward to its restrained position where it is held stationary,

FIG. 4 is an illustration of the measured irregularities along the inner surface of a commercially produced tube, as compared with the measured irregularities in a straight bore tube drain according to the invention, and

FIG. 5 is -a curve representative of the effect of mandrel extension on the straight bore tube and the dis placement of the wave to the outside surface of the 1 tube.

In the production of rotary pierced seamless tubing, the action of the cross rolling over a piercing tool results in twisting of the metal fibers so that any eccentricity which may occur will be arranged in one or more spiral patterns down the length of the tube. The eccentricity of the wall results in displacement between the centerlines ofthe inner and outer surfaces. The normal tube making operations react on the tube with the spiral of heavy wall juxtapositioned with a spiral or light wall down its length so that inside and outside surfaces have roughly equal spiral waves as shown in FIG. 1A.

reduction in, the tube surface irregularities is desirable.

For. example,'when the tubes are used to form the cylinder of a power piston, the inner surface of the tube should have this spiral wave reduced to less than onehalf the value normally found in a routinely processed tube. Such conditioned surface is attained by honing with special long'hones which cut to the bottom of the spiral wave, thereby eliminating it. .Any reduction in the normally encounteredinternal spiral wave in the tube will reduce or eliminate the cost of this, special honing and'such reduction is of great economic advantage t the producer of power pistons.

Measurement of the amount, location and pattern of curvature, wave, or departure'frorn straightness in the inner surface of commercial tubes has, in'th e past, been laborious to set up and perform and has required costly equipment and relatively high degreeof skill.-Practically the only known method required an extensive surface plate with instrumentation involving a stand supporting a dial indicator on the end of abeam extending dicated by curve A. It will be note d that-the measured irregularities amounted to as much as plus and minus twenty one-thousandths of an inch in l5 inches. It .will also be noted that the curve does not show a steady sinusoidal variation, but rather a dominant wave, modulated by the addition of a weak wave of different pitch alternately augmenting and then decreasing the dominant wave depending on the relative phase of the two components. It will be appreciated that the curve A is drawn from readings taken along a single line of the tube surface. A comparable curve could be drawn for any line parallel to the tube axis and rotationally displaced on the inner surface of the tube. However, the peaks and valleys of such other measurements would 'be displaced longitudinally of the tube length to indicate the spiral nature of such irregularities.

As hereinafter more completely described, curve B on FIG. 4 indicates the extent of irregularities on a straight bore tube when manufactured in correspondence with the present invention. It will be noted on this curve the maximum variation is of the order of five one-thousandths of an inch in the length measured. The comparisonof curves is typical of the results attained by the use of the present invention in cold drawing a tube.

It is generally known that the eccentricity and resulting inside surface wave in a rotary-pierced tube-follows a spiral pattern. Actually, eccentricities resulting from several initiating factors have differing pitches and even opposite directions of rotation, but most of these are negligible compared with the dominant eccentricity spiral which'is oriented opposite in direction and of shorter pitch than the seam spiral which is well known in .the art. While there are variations resulting from the amount of elongation given in the hot rolling, reeling and sizing operations which follow the rotary piercing, it hasbeen found that these will not affect the resultant pitch by more than plus or minus 20 percent, so that the geometry of the straight bore cold draw pass can be designed to be effective with a factor or safety. As an example of this consistency, even with mills of different design, it has been found that the average pitch of the dominant inside surface wave is l7 inches for cone-roll type piercersand l 5 inches for barrel-roll type Mannesmann piercers. This is due to the fact that the dominant eccentricity spiral does not result'from the external rollcontours but rather from rolling of the piercing tool inside the hollow during the wall reduction phase of piercing. At this point, the piercing tool has a shorter perimeter than'that of the inside of the hollow, so that one rotation of the piercing tool does not bring itback to the, same angular position inthe tube, and any discrepancy in too] shape hits thewall along a spiral path. Since the perimeter discrepancy is I roughly the .same even in mills of different roll design,

manner and an elongated cylindricaimandrel l2 in- Asshown in FIG. 2, the tube 10 and mandrel 12 have been inserted in the port of the die 17 with the pointed end 11 of the tube engaged by the jaws 20 of a conventional drawing device 21 of the drawbench. The drawing device 21 pulls the tube 10 through the die 17 with the'mandrel l2 advancing through the die to extend downstream in a tube drawing direction, as shown in FIG. 3. The mandrel 12 is maintained in its relationship to the die 17 through the remainder of the tube drawing procedure. The drawn tube 10 leaving the die 17 will have a reduced outside and inside diameter and wall as determined by the die and mandrel dimensions. When cold drawn within the limitations of die, mandrel and tube dimensions hereinafter described, the resulting product will have a straight bore with a minimum of irregularities on its inner surface.

Fundamental to the understanding of the invention is the concept of stiffness ratio. Referring to FIG. 3, the

mechanics of the tube l0'.and the mandrel 12 where straight andan inner surface centerline ISCL which has v a spiral ,wave. The mandrel, on the other hand, is

straight and attempts to make the interface between tube and mandrel, and consequently the inner surface of the, tube, be a straight cylinder. This configuration is shownin FIG. 18 where the inner surface centerline ISCL is straight, the wall mid-section MS has a spiral wave and the ;total discrepancy of spiral eccentricity appears on the outside surface. Thetube exerts forces which tend to make the interface spiral and the man-.

drel exerts forces to make the interface straight, so this interface configuration will depend on the relative stiffness of the tube and the mandrel, and assuming both have the same modulus of elasticity will depend on the moment of inertia of the tube section and the moment serted in the pointed tube with mandrel-end l3 abutting the inner surfacel4 of the pointed tube-end.,The mandrel 12 is mounted" by conventional structure 'on the end of a mandrelbar 15, which in turn is supported in a conventional bar holder 16.The holder is constructed to permit controlled advance of the bar during the start of the draw, shown in FIG. 2 so as to assure engagement of the mandrel by the tube, followed by restraint of the bar when the mandrel reaches the desired protrusion through the throat of the drawing die where it holds it stationary, as shown in FIG. 3.

of inertia of the mandrel section. Straight-forward calculations show the ratio of tube stiffness to mandrel stiffness to be SR.= (OD /ID) 1. In this-relationship, the OD is the outside diameter of the drawn tube (10) while the ID is the inside diameter of the drawn tube which is also the outside diameter of the'mandrel (l2). Toillustrate the importance of stiffness ratio, if atube were drawn to V4 inch OD and V4 inch ID, the quarter= inch wire would merely bend to conform to the inside configuration assumed by the tube, and would vexertalmost no inside straightening effect. In this case,.the

tube would be times as stiff as the mandrel. The results in this example are quite obvious, but it was desired to determine atwhich value .of stiffness ratio would the, straightening effect die out. 'A test series involving pieces of tubing ranging in size from 3. l 87 inch OD X02 16 inch wall to 8.250'inch OD X1383 inch wall andhaving stiffness ratios from 0.78 to 7.0

was conducted, exploring the effects of stiffness ratio,

mandrel extension beyond the die throat and percentof the mandrel beyond the die throat. At a stiffness ratio of about 3, the proportionality constant increases in a ratio of about 3:1 and remains constant. For example, with the lubrication practices of the tests, the forces on a 2% inch mandrel increase at a rate of 1000 pounds per inch of extension for stiffness ratios from 0.78 to 1.15, and increase at a rate of 3200 pounds per inch of extension for stiffness ratios of 2.9 to 7.0. On a 5% inches mandrel, these ratios of increase would be 2500 pounds per inch of extension at stiffness ratio of 0.78 to 1.15 to 7800 pounds per inch of extension at a stiffness ratio of 2.98. The effectiveness of inside straightening drops off somewhat at a stiffness ratio in the order of 3, though some improvement was apparent at 4, if the mandrel is extended well downstream. It may be difficult to understand how substantial straightening can be accomplished at any value of stiffness ratio above SR=l. This is explained by the fact that the tube is under tensile stresses of from 18,000 pounds per square inch to 57,000 pounds per square inch as it leaves the die. Under this condition, the

added bending stress will undoubtedly strain the tensile fibers beyond the yield point so that formulasinvolving constant modulus of elasticity are not strictly true, and the tube yields more readily than would be calculated.

The second factor in the successful practice of this process involves the distance the mandrel is extended beyond the die throat. If the extension is too short, the mandrel does not provide appreciable straightening because it does not contact enough of the wave to produce enough deflection for a set. FIG. 5 shows how the increasing contact length of the mandrel improves the transference of spiral wave from the inside to the outside surface as the mandrel contacts a greater portion of the pitch of the wave. FIG. Sexpresses curvatures as thousandths of an inch in fifteen inches departure from straight based on the measurements made by the measuring device previously mentioned as ordinates and length of contact between mandrel and tube as a percentage of pitch length of wave spiral as abscissa. Each'point represents the average of from 12 to 24 tubes over the full size range and having stiffness ratios from 0.78 to 2.98. The left end of FIG. 5 represents normal drawing practice with almost no mandrel extension, and producing a tube with roughly equal spiral waves on the inside and outside surfaces. As the mandrel contacts a greater portion of the wave, the effectiveness increases, reaching an almost constant value at about one-third of the pitch length where the inside wave has been reduced to approximately 33 percent of the value resulting from normal drawing practice. Visualizing the cross section of the tube wall as having a sinusoidally varying section, the mandrel would be contacting of the sinusoidal wave, or 60 each side of a peak.

There are also limitations on the length of mandrel extension which may be used. As length increases, the mandrel bar forces increase, particularly in the higher stiffness ratios where mandrel deflection aggravates the buildup of mandrel bar forces. This force problem is accentuated by a peak break-away force amounting to percentto percent of steady state forces under good lubricating conditions, and may reach 200 percent in case of lubricant failure. In an actual test with a mandrel extension of 19 inches, a mandrel bar was broken on the drawbench. Fortunately, these limitations do not prevent practical application of the process. The pitch of the spiral wave of the tube as it comes from the cold draw operation is elongated to approximately l25 percent of the 15 to 17 inch pitch as received from the hot rolling mill. This will result in a pitch of approximately 20 inches or would require a mandrel extension of 6 to 7 inches. It has been found to be quite practical to use these mandrel extensions and even increase them to provide slightly greater correction and provide afactor of safety. in case a tube should be supplied that had been given a previous cold drawing pass resulting in a longer than normal spiral wave pitch.

As used in the claims, a straight bore tube is defined as a tube of circular cross section which has been processed to straighten the inside surface by transferring a substantial portion of the surface wave resulting from the spiral orientation of eccentricity produced by rotary piercing to the outside surface, thereby substan tially reducing the spiral wave on the inside surface.

I claim:

1. A method of forming a straight bore tube by cold drawing wherein a tube having a mandrel of substantially uniform cross-section therein is drawn through a die, the steps of extending the mandrel through the die with the tube during the initial cold drawing of the tube through the die, controlling the position of the mandrel to fix its position within the die and limiting the location of its-downstream end during the remainder of the tube drawing procedure wherein the stiffness ratio between the drawn straight bore tube and the mandrel is coordinated with the extension of the mandrel downstream of the die to force displacement of irregularities from the internal surface to the external surface of the drawn tube. A

2. A method according to claim 1 wherein the stiffness ratio between the drawn straight bore tube and the mandrel is of the order of 3. 

1. A method of forming a straight bore tube by cold drawing wherein a tube having a mandrel of substantially uniform crosssection therein is drawn through a die, the steps of extending the mandrel through the die with the tube during the initial cold drawing of the tube through the die, controlling the position of the mandrel to fix its position within the die and limiting the location of its downstream end during the remainder of the tube drawing procedure wherein the stiffness ratio between the drawn straight bore tube and the mandrel is coordinated with the extension of the mandrel downstream of the die to force displacement of irregularities from the internal surface to the external surface of the drawn tube.
 2. A method according to claim 1 wherein the stiffness ratio between the drawn straight bore tube and the mandrel is of the order of
 3. 